Heretofore veneers for plywoods have been cut from logs having properties suitable for the production of plywoods. Logs with the suitable properties are mostly obtained in Asian torrid zones. The suitable properties include the capability of being readily bonded to wood with an adhesive material, the capability of drying in a relatively short period of time, the property of not emitting an offensive odor, and in particular the property of being substantially composed of straight fibers.
Usually an odd number of veneers are used for the production of a plywood. For the production of a three-layer plywood, two veneers (of the same thicknesses) are bonded to the opposed sides of a core veneer such that the fibers of the two outer veneers run at right angles to the fibers of the core veneer. From a different point of view, the fibers of the two outer veneers run in the same direction. Similarly a five-layer plywood consists of a core veneer whose fibers run in one direction, two outer veneers bonded to the opposed sides of the core veneer such that the fibers of the two outer veneers run in the same direction, making right angles to the fibers of the core veneer, and two outermost veneers bonded to the two outer veneers, respectively, such that the fibers of the two outermost veneers run in the same direction, making right angles to the fibers of the two outer veneers. Also, a plywood is produced in such a manner that a neutral plane of the plywood, i.e., the plane of the plywood where the plywood is not subjected to expansion and contraction, is located at the middle of the thickness of the plywood.
Heretofore, however, if veneers with unstraight fibers are used, a normal plywood has not been produced since, as a matter of course, it is not possible to place such veneers on one another such that the fibers of the adjacent ones of them make right angles with each other. The use of such veneers have resulted in a curved or waved plywood. Needless to say, veneers with unstraight fibers come from logs with unstraight fibers. Therefore, logs with unstraight fibers hitherto have not been used for the production of plywoods.
However, since a great deal of wood resources have been exploited in the past, there is now the necessity to use even logs with unstraight fibers for the production of plywood. To meet this necessity, the applicant proposed veneer lathes disclosed in Japanese Patent Publications Nos. 56-16729 and 59-19007 and a rotary lathe disclosed in Japanese Patent Publication No. 61-21805. However, these lathes in themselves are not capable of producing veneers usable for the production of uncurved and unwaved waved plywoods, from logs with unstraight fibers.
Unstraight fibers include diagonal fibers, spiral fibers, interlocked fibers and wavy fibers.
Some of the reasons why the use of veneers with unstraight fibers results in a curved or waved plywood will now be described.
One of the reasons resides in a behaviour of a veneer. The mechanical strength of a veneer is much smaller in its direction perpendicular to its fibers than in its direction of the fibers. Therefore, for example, if a veneer absorbs a large amount of moisture, the veneer expands much more in its direction perpendicular to its fibers than in its direction of the fibers. Thus, with a veneer having unstraight fibers, different portions thereof with fibers running in different directions expand or contract in different directions. For this reason, if such veneers are used to produce a plywood, a curved or waved plywood may result.
Another reason relates to a behaviour of an adhesive material. For the production of a plywood, veneers are bonded together with an adhesive material. A typical adhesive material is a liquid, thermosetting adhesive material containing synthetic resin (such as urea resin, melamine resin or phenolic resin) or denaturalized, copolycondensed or mixed resin obtained by using the synthetic resin. Such a type of adhesive material contains a large amount of solvent, such as water. In addition, where necessary, an additional solvent (such as water), an extending agent (such as wood dust or wheat flour), a curing agent (such as ammon chloride), a filler, a thickening agent and/or a plasticizer is added to the adhesive material. Then, the adhesive material is applied to a veneer, and the veneer is bonded to another veneer while the whole is heated. The adhesive material is thus hardened as part of the solvent contained in the adhesive material is absorbed, together with part of the other portion of the adhesive material, in the veneers and the remaining part of the solvent evaporates from the adhesive material. Consequently the adhesive material shrinks considerably.
After the adhesive material has been applied to the veneer, the veneer swells. But as the adhesive material shrinks, the veneer also shrinks. However, the veneer shrinks only in its weaker direction, i.e., in its direction perpendicular to its fiber. Also, the veneer swells once again as it absorbs the vapor of the solvent produced from the adhesive material. Therefore, the veneer is not capable of shrinking in exactly the same manner as the adhesive material. Consequently, the veneer is considerably compressed.
Thus, the veneer shrinks to a certain degree in its direction to its fibers. However, if the veneers are bonded together such that a product (plywood) has a neutral plane at the middle of the thickness of the product, the shrinkage of the veneers does not result in a curved or waved product unless there are substantial differences in the directions and/or degrees of shrinkages between the corresponding portions of the upper half and lower half into which the product may be divided with respect to the neutral plane thereof. If veneers with straight fibers are used to produce a plywood, a curved or waved plywood does not result since there are no substantial differences in the directions and degrees of shrinkages between the corresponding portions of the symmetrical pair of veneers. But, if veneers with unstraight fibers are used, a curved or waved plywood result since there are substantial differences in the directions of shrinkages between different portions of each such veneer with the result that the directions and degrees of shrinkages of the different portions of one of the symmetrical pair of veneers differ substantially from the directions and degrees of shrinkages of the corresponding different portions of the other veneer.